Selective stimulation of human platelet lipoxygenase product 12-hydroxy-5,8,10,14-eicosatetraenoic acid by chlorpromazine and 8-(n,n-diethylamino)-octyl-3,4,5-trimethoxybenzoate

Abstract

Stimulation of human platelets by thrombin and by the Ca2+ ionophore A23187 leads to a rapid Ca2+-dependent activation of phospholipases that release membrane-bound arachidonic acid for oxidation by a cyclooxygenase and lipoxygenase enzymes into so-called eicosanoids. Chlorpromazine and the intracellular calcium antagonist 8-(N,N-diethylamino)-octyl-3,4,5-trimethoxybenzoate (TMB-8) inhibited the release of eicosanoids, as estimated by a quantitative glass capillary-gas chromatography analysis. TMB-8 was more efficient for thrombin- than for ionophore-induced eicosanoids liberation. Chlorpromazine, the more potent inhibitor, was active at the same concentration against either inducer. The reduction of oxidative metabolism by the cyclooxygenase pathway was more pronounced than reduction in the lipoxygenase pathway. When exogenous arachidonic acid was added to the platelets, both drugs stimulated selectively the production and the formation rate of 12-hydroxy-5,8,10,14-eicosatetraenoic acid by a factor of 2-2.5 in the absence of variation of cyclooxygenase products. Therefore, the stimulation of the lipoxygenase metabolite by the two drugs was obtained with both endogenous and exogenous arachidonic acid. This selective stimulation by drugs of a lipoxygenase product in the absence of inhibition of cyclooxygenase is the first reported of this type and suggests a differential control for the two oxidation enzymes. These findings emphasize the importance of a simultaneous quantitative analysis of both oxidation pathways.